A typical battery is made up of individual cells connected in series (or sometimes in parallel) to form a stack, where only a first terminal on one side of the stack and a second terminal on the other side of the stack are available for connection to devices external to the battery. In this configuration, a current flowing into and out of the stack is equal through each cell in the series. Each cell may include metal (e.g. metallic plates) that stores a charge and may have a chemical electrolyte that acts upon the metal to enable increasing or decreasing the charge. A resting voltage potential of each cell, a minimum charge level, and a maximum charge level of each cell may depend on a particular chemistry of the cell. Operating the battery outside of the resting voltage potential, the minimum charge level, and the maximum charge level may diminish cell performance.
Common methods for charging batteries include outputting from a charging device a voltage or charge current according to a charge level of the battery. If the battery is already close to fully charged, a trickle charge might be applied, or if the battery charge is very low and needing to be brought back to a threshold level before applying a full charge, a kind of restorative charge might be applied and during normal charging fast or slow charge profiles depending on the needs of the battery and the wishes of the user might be applied.
Because battery chemistries tend to have needs unique to their physical make up it is most common for charging systems to be configured for one chemistry over others. An example of this is vehicle lead acid battery systems where charge and discharge levels, rate of charge and determination of battery state are largely fixed at lead acid levels and therefore introduction of another battery chemistry can complicate the proper care and maintenance of the battery.
In order to make a high performing system most cost effectively, current vehicle systems are configured to support a single kind of battery chemistry. Though advances in battery chemistry may make new battery types more ideal for particular applications, these systems are unable to adapt to the new battery types. For example, most lead acid batteries have a charge termination (where the battery is fully charged) of around 13.8 volts and a lithium ion polymer based battery might have a charge termination voltage of 12.6 volts. A lithium iron phosphate battery might have a charge termination of 13 volts. Substitution of a battery within a particular system with a battery for which the system was not designed may result in the battery being overcharged and potentially damaged.
As another example, discharge termination is also different for different types of batteries. Most battery chemistries have a low voltage threshold, where current output rolls off considerably, and once the battery has been discharged beyond the threshold, it provides much less current. If the battery continues to discharge battery depletion may occur. In lead acid battery systems this roll off occurs at over ten volts whereas in the lithium based systems this roll off might occur at roughly 12.6 volts.
Over-current or over-capacity discharge can greatly reduce battery life. Preventing over-current or over-capacity discharge by turning off battery output can greatly reduce or prevent damage to the battery. A considerable downside to this is that most vehicle charging systems are inductive and removing the battery from the system while it is charging can create a load dump, causing a large output spike which may damage the charging system and other critical vehicle electronics. An unfortunate side-effect of turning off battery output to prevent over discharge is that vehicle system electronics, even if they remain undamaged, also have their power cut and will no longer be able to operate.
Further, most vehicle systems have their battery connections only at the terminal ends of the battery pack and therefore individual cell charge levels are balanced solely by the similarity of cell behavior (similar cells charge and discharge at a similar rate). However individual cells within a battery may have differences due to manufacturing processes and differing deterioration rates. These differences may cause diverging charge efficiency on cells within the same stack.
For battery stacks with only one pair of terminal connections the decision for terminating charge is traditionally determined for an end voltage (over the entire battery stack) whereby the total of all cell charge levels make up a final charge threshold for the entire stack. If one or more cells accumulate charge from a charging current at a slower rate than its companion cells in the stack, other cells may receive a higher charge level than intended. For instance if a 12 volt lead acid battery has a nominal cell voltage of 2.1 volts and six cells in a stack, where each cell is fully charged at 2.3 volts, then a charging system might charge the battery to 13.8 volts (6 volts*2.3 volts). If one cell in the stack has a lower voltage than the rest of the cells then the rest of the cells will have been charged to a higher level than intended. For example, if three cells accepted a charge at a slower rate resulting in charge levels of 1.8 volts on average, then the other three cells will have charged to an average of 2.8 volts each (3 cells*1.8 volts+3 cells*2.8 volts=13.8V). Hence, three cells may be overcharged and three cells may be undercharged. When one or more cells does not charge with the same efficiency as the other cells then, after repeated discharge and charge cycles, a charge imbalance may become amplified and significant. This may result in one or more cells becoming charged over its rated level or discharged below its rated level, thereby damaging the cell or its companion cells and diminishing the overall stack efficiency.
Battery manufacturers attempt to make batteries with cells of equal behavior to help minimize cell charge imbalance, but aging of the electrolyte and metal panels, damage through heat or cold, and/or over and under charging can alter cell behavior over time resulting in unbalanced charging. Cells age differently over time and since current charging systems apply current equally through all the cells in the series stack, the battery usefulness may be decreased in accordance with the output of the weakest cell. Other drawbacks and disadvantages of current systems and methods also exist.